Studies designed to follow possible deteriorations in DNA repair mechanisms with the age will be undertaken with the following leporine tissues: (1) Skin fibroblasts in vitro; (2) Retinal photoreceptors in situ; (3) Lymphocytes in situ. The principal mechanisms to be studied are those which rejoin DNA strand breaks following gamma-irradiation; and the analytical method used will be reorienting gradient zonal centrifugation in alkaline sucrose. Such studies will be complemented by histological and histopathological examinations.